Anaesthesia Coffee Break How does morbid obesity affect washout of volatile anaesthetic?
Feb 1, 2021
Explore the complexities of how morbid obesity affects the washout of volatile anesthetics. The discussion covers definitions of obesity categories based on BMI and the significance of fat distribution. Learn about washout determinants including partial pressures and time constants. The hosts compare desflurane and isoflurane, and debate the role of cardiac output in anesthetic elimination. Practical tips for agent selection and extubation techniques in obese patients are shared, emphasizing the importance of safe intraoperative strategies.
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Obesity Is Not Just Fat
- Morbid obesity's extra mass is about two-thirds fat and one-third organ/muscle, so not all excess weight is poorly vascularized fat.
- Increased muscle/organ mass provides a highly vascularized reservoir that meaningfully affects volatile kinetics.
Washout Is About Compartments And Time Constants
- Washout depends on partial pressures in muscle and fat compartments and on routes of excretion.
- Time constants (solubility, compartment size and blood flow) and amount delivered (Fi and duration) determine those partial pressures.
Fat Time Constants Are Very Long
- Tissue time constants differ hugely: muscle time constants are minutes, fat time constants are hours.
- Fat contribution to clinical washout becomes significant only after very prolonged anesthesia (tens of hours).